ALEPPO - A bomb exploded in central Damascus on Wednesday close to several military buildings and a hotel housing United Nations observers, wounding three people and sending a pillar of black smoke into the sky above the Syrian capital.

Syrian Deputy Foreign Minister Faisal Mekdad said none of the U.N. monitors was hurt in the explosion, which occurred exactly four weeks after a bomb killed four of President Bashar al-Assad's top security officials.

"This is another criminal operation which proves the (extent of the) attack which Syria has been exposed to and the criminal and barbaric nature of those who carry out these attacks - and their backers in Syria and abroad," Mekdad told reporters at the scene where fire-fighters were dousing a smouldering fuel tanker.

The bomb, in a car park behind the hotel, blasted open the fuel truck when it detonated at 8.30 am (0530 GMT). A row of white U.N. vehicles parked nearby was covered in ash and dust.

Although the explosion occurred close to the hotel, it was not clear what the target had been. The area is home to a Syrian army officers' club and a building belonging to the ruling Baath Party. It is also not far from the army command.

Assad's troops launched a counter-offensive last month against rebels who seized several districts of Damascus and swathes of the country's biggest city, Aleppo.

The violence in Syria, where opposition sources say 18,000 people have been killed since the uprising against Assad erupted in March last year, has divided regional and world powers and blocked diplomatic efforts to end the crisis.

Leaders of Muslim countries are expected to suspend Syria's membership of the Organization of Islamic Cooperation at a summit in Mecca on Wednesday, despite the vocal objections of Iran, Assad's main ally and a regional Shi'ite Muslim power.

The largely symbolic decision by the 57-member body, which requires a two-thirds majority, will expose the divisions in the Islamic world over how to respond to civil war in a country that straddles the Middle East's main sectarian fault line. Syria's Sunni majority is at the core of the revolt. Its leadership is dominated by Assad's Alawite sect, an offshoot of Shi'ite Islam.

CONCILIATORY GESTURE

In an apparently conciliatory gesture, Saudi King Abdullah welcomed leaders to the summit with Iran's President Mahmoud Ahmadinejad at his side. Abdullah and Ahmadinejad were shown on Saudi state TV talking and laughing together.

"It was a message to the Iranian nation and, I assume, to the Saudi people, that we are Muslim and we have to work together and forget about our differences," said Abdullah al-Shammari, a Saudi political analyst.

Syrian rebels are backed by Sunni-ruled states such as Saudi Arabia and Qatar as well as Turkey, while Iran supports Assad.

Assad's former Prime Minister, Riyad Hijab, a Sunni who defected this month, made his first public appearance on Tuesday since he fled, telling a news conference in Jordan that Assad controls less than a third of Syria and his power is crumbling.

"The regime is collapsing, spiritually and financially, as it escalates militarily," he said. "It no longer controls more than 30 percent of Syrian territory."

Hijab was not in Assad's inner circle, but as the most senior civilian official to defect; his defection after two months in the job was a blow to the president.

Hijab did not explain his estimate of the territory still controlled by Assad, whose military outnumbers and outguns the rebels fighting to overthrow him. The army is battling to regain control of Aleppo, Syria's biggest city, after retaking parts of Damascus that were seized by insurgents last month.

Curbs on media access make it hard to know how much of Syria is in rebel hands, but most towns and cities along the country's backbone, a highway running from Aleppo in the north to Deraa in the south, have been swept up in the violence, and Assad has lost swathes of land on Syria's northern and eastern border.

Hijab's defection prompted Washington to announce on Tuesday that it was removing him from a list of Syrian officials targeted by financial sanctions.